The invention relates to a cryopump, or cryogenic pump, having surfaces exposed to the gases to be pumped, some of these surfaces being intended for the adsorption of the gases while others are not.
Refrigerator-operated cryopumps are known from German patent publication DOS No. 26 20 880, 28 21 276 and 30 38 415. They comprise three surfaces intended for adsorption of the various types of gases. The first surface is in good thermal contact with the first stage of the refrigerator and has a substantially constant temperature between 60 and 100 K, depending on the type and output of the refrigerator, with a low temperature gradient. A metal having appropriate heat-conductivity properties is selected as material of construction. These surfaces, which may include the surface of a baffle that protects the lower-temperature pump surfaces from incident heat radiation, serve mainly for the deposition of water vapor and carbon dioxide by cryocondensation. Cryocondensation occurs when gases impinge on a precoated homogeneous surface and condense to the liquid or solid phase. The binding forces are of physical nature, and the binding energy corresponds to the heat of vaporization.
The second surface is in thermal contact with the second stage of the refrigerator. It is likewise a metal surface and is intended for the removal, by cryocondensation and cryotrapping, of hydrogen, argon, carbon monoxide, methane and halogenated hydrocarbons, for example. Cryotrapping designates the process in which lower-boiling and therefore more difficulty condensable gases impinge on a precoated surface simultaneously with more readily condensable gases, the more difficulty condensable gases being incorporated in the steadily growing condensate film of the more readily condensable gases.
The third surface is also at the temperature of the second stage of the refrigerator (or at a correspondingly lower temperature in the case of a three-stage refrigerator) and is covered with an adsorbent (activated carbon or the like). It is essentially on this surface that the cryosorption of lighter gases such as hydrogen, helium and neon is to take place. Cryosorption occurs when gases impinge on an uncoated, heterogeneous surface and are bound by unsaturated residual valences of the interfacial atoms of the surface. These surfaces are arranged in such a way that they can be reached by the light gases only by "detours". The heavier gases are practically unable to diffuse into spaces with cryosorption surfaces which can be reached only by a circuitous route. They will condense on the readily reachable cryocondensation surfaces. Premature contamination of the adsorbent with heavy gases is thus prevented. The pumping activity for light gases is preserved for a longer period of time.
In addition to these surfaces serving for the adsorption of the gases to be pumped, prior-art cryopumps comprise surfaces which are not intended to adsorb gases. These are the exteriors of the cylindrical tubes in which the displacers of the refrigerators move and which extend from the foot of the pump to the first stage of the refrigerator and between the first and second stages.
In tests conducted with cryopumps of the type described, and also in using such pumps in sputtering systems, the problem has been encountered time and again that the pumping times of these pumps are unduly long, that is to say, the pumps take a relatively long time to attain the desired low pressures, particularly when the pressure in the pump during a sputtering operation momentarily rises to a relatively high absolute-pressure level (e.g., 1.times.10.sup.-2 millibars). Moreover, pressure fluctuations would occur during operation for which there was no explanation at first. The object of the invention thus was to eliminate these problems. This object could be accomplished only after the inventors had come to the following conclusions concerning the cause of the described problems:
With prior-art cryopumps, it is unavoidable, especially after an extended period of operation, that gases should condense also on surfaces which are not intended to adsorb gases. Because of the temperature gradient, there are surfaces in these pumps which are at such intermediate temperatures that gases having specific physical properties (argon, for example) are adsorbed at elevated pressures and then are desorbed as the pressure drops. This may happen to such a degree that the slowly desorbing gases will have a pressure-determining effect, that is to say, will prevent the desired pressure reduction, for a relatively long time.
Moreover, it has been found that temperature variations occur on these surfaces which are tied to the cycle of motion of the displacer that moves periodically therein. Especially in the case of gas mixtures adsorbed by cryocondensation or cryotrapping, these temperature variations give rise to local desorptions, adsorptions and rearrangements of gases which cause undesired pressure fluctuations in the vacuum space.